This only applies if you signed up in the iOS app using your Apple/iTunes account.
If you've used the Restore Purchases option but it only seems to temporarily put you back on Player+, or if it changes you back when you visit the Plan page in the Settings Menu, then check you are signed into the STV Player with the account you originally used to sign up to Player+.
Your subscription requires both the Apple Account and the STV Player account details to match the original ones used to sign up , if only the Apple Account matches then although your subscription can be verified, it cannot be applied to the Player account, so will eventually revert back.
You can check which account is currently signed in by going to Settings > Account and checking the email address. You can also Sign Out from the Settings section and then sign in with the correct account and then you can Restore your Purchase and have it remain on your account.